The problem of controlling static charge during plastic web manufacturing and transport is well known. Generation and uncontrolled discharge of electrostatic charge can cause a number of serious problems including safety hazards. In the field of imaging, particularly photography, the accumulation of charge on film or paper surfaces leads to the attraction of dirt, which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or “static marks” in the emulsion. The static problems have been aggravated by increase in the sensitivity of new emulsions, increase in coating machine speeds, and increase in post-coating drying efficiency. The charge generated during the coating process may accumulate during winding and unwinding operations, during transport through the coating machines and during finishing operations such as slitting and spooling.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically-conductive “antistatic” layers into the support structure. Typical location of an antistatic layer is an external surface, which comes in contact with various transport rollers. For imaging elements, the antistatic layer is usually placed on the side of the support opposite to the imaging layer.
A wide variety of electrically-conductive materials can be incorporated into antistatic layers to produce a wide range of conductivities. These can be divided into two broad groups: (i) ionic conductors and (ii) electronic conductors. In ionic conductors charge is transferred by the bulk diffusion of charged species through an electrolyte. Here the resistivity of the antistatic layer is dependent on temperature and humidity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts), described previously in patent literature, fall in this category. However, many of the inorganic salts, polymeric electrolytes, and low molecular weight surfactants used are water-soluble and are leached out of the antistatic layers during processing, resulting in a loss of antistatic function. The conductivity of antistatic layers employing an electronic conductor depends on electronic mobility rather than ionic mobility and is independent of humidity. Antistatic layers which contain conjugated polymers, semiconductive metal halide salts, semiconductive metal oxide particles, etc., have been described previously. However, these antistatic layers typically contain a high volume percentage of electronically conducting materials, which are often expensive and impart unfavorable physical characteristics, such as color, increased brittleness and poor adhesion, to the antistatic layer.
A vast majority of the prior art involves coatings of antistatic layers from aqueous or organic solvent based coating compositions. For photographic paper, typically antistatic layers based on ionic conductors, are coated out of aqueous and/or organic solvent based formulations, which necessitate an effective elimination of the solvent. Under fast drying conditions, as dictated by efficiency, formation of such layers may pose some problems. An improper drying will invariably cause coating defects and inadequate adhesion and/or cohesion of the antistatic layer, generating waste or inferior performance. Poor adhesion or cohesion of the antistatic layer can lead to unacceptable dusting and track-off. A discontinuous antistatic layer, resulting from dusting, flaking, or other causes, may exhibit poor conductivity, and may not provide necessary static protection. It can also allow leaching of calcium stearate from the paper support into the processing tanks causing build-up of stearate sludge. Flakes of the antistatic backing in the processing solution can form soft tar-like species, which, even in extremely small amounts, can re-deposit as smudges on drier rollers eventually transferring to image areas of the photographic paper, creating unacceptable defects.
Moreover, majority of antistats on current photographic paper products lose their electrical conductivity after photographic processing due to their ionic nature. This can cause print sticking after drying in the photoprocessor, and/or in a stack.
In U.S. Pat. Nos. 6,197,486 and 6,207,361, antistatic layers have been disclosed which can be formed through the (co)-extrusion method thus eliminating the need to coat the support in a separate step and rendering the manufacturing process less costly.
When placed as an external layer, the antistatic layer may be required to fulfill additional criteria depending on the application. One such criterion is the conveyance of the web through many different types of equipment. For photographic paper, for example, the web must convey through various machines, which involve base making, sensitizing, slitting, photographic printing, processing, finishing, etc. Efficient transport of such products necessitates a tight control of the roughness of the external layer. As disclosed in U.S. Pat. No. 6,022,677, photographic papers with a backside roughness average, Ra, of less than 0.3 μm cannot be efficiently transported in the photoprocessing equipment, as many transport problems will occur. Transport problems such as, scratching, machine jams, and poor print sticking will occur with backside Ra of less than 0.3 μm. In majority of color paper products, such a desirable roughness on the backside of the paper can be achieved by casting polyethylene against a rough chilled roll. Photographic papers made in this manner are very efficiently transported through photoprocessing equipment. However, polyethylene coated photographic papers, when exposed to varying humidity, may experience serious curl that can interfere with the viewing of images. A solution to this curl problem is proposed in U.S. Pat. No. 5,902,720, through the use of biaxially oriented polyolefin sheets, which unfortnately provides a backside roughness of Ra less than 0.23 μm.
In the final image format, it is common for consumers to write personal information on the backside of the images with pens, pencils, and other writing instruments. Photographic papers that are smooth on the backside are more difficult to write on. There is also a desire to print information from Advanced Photo System negatives onto prints made from these negatives. Therefore, there is a need for color prints to receive printing and writing on their back. There remains a need for photographic papers that are sufficiently rough so that writing or printing on the backside of the photographs can be easily accomplished.
During the manufacturing process for photographic papers, it is a requirement that silver halide emulsion coated paper be handled and transported in roll form. In roll form, the backside of the photographic paper is in contact with the silver halide image forming layer. If the roughness of the backside exceeds 2.54 μm, the image forming layer would begin to become embossed with the surface roughness pattern while in the roll form. Any customer perceived embossing of the image forming layer will significantly decrease the commercial value of the image forming layer. Furthermore, silver halide emulsions tend to be pressure sensitive. A sufficiently rough backside, in roll form, would begin to also destroy the commercial value of the image forming layer by developing the silver emulsion with pressure from the surface roughness of the backside. There remains a need for a photographic paper that has a backside roughness less than 2.54 μm so that photographic paper can be conveniently wound and stored in roll format.
In the formation of reflective receivers for digital imaging systems such as Ink Jet and Thermal Dye Transfer, there is a need to reduce the curl of the image. Lamination of a high strength biaxially oriented polyolefin sheet to the backside of the image does improve the curl over the common practice of extrusion coating a layer of polyolefin. Reflective receivers for digital imaging systems that have a smooth backside will cause transport problems in the various types of printers that are common in digital printing. Transport difficulties resulting from a smooth backside could cause unacceptable paper path jams, scratches on the image, and failure to pick the receiver from a stack. The latter problem can be further aggravated by electrostatic attraction between contiguous sheets. For ink jet and thermal dye transfer receivers it would be desirable if a backside surface could be formed with a surface roughness greater than 0.30 μm with antistatic characteristics to allow for efficient photoprocessing.
Photographic papers with biaxially oriented polyolefin sheets with a backside Ra between 0.3 μm and 2.0 μm are proposed in U.S. Pat. Nos. 6,022,677 and 6,030,742. The roughness of the backside surface is claimed to have been achieved either through the use of particulate addenda or by a mixture of incompatible block copolymers of polyethylene and polypropylene. Although these polyolefin sheets possess the desired roughness, they lack electrical conductivity, and therefore require separate antistatic layers for effective charge control.